The present invention relates to methods and apparatus for indirectly cooling a fluid by evaporation and more specifically to a method and an apparatus for cooling air, or a product other than air, to substantially the dew point temperature of the air used in the evaporative cooling.
Indirect evaporative cooling has been used for many years to cool high temperature fluids down to near the wet bulb temperatures for commercial and industrial processes such as refrigeration systems. The use of indirect evaporative cooling for direct air conditioning systems has been commercially available but never commercially viable as proven by the lack of available products on the market at costs that reflects their effectiveness.
The commercially available indirect evaporative cooling systems use a two-step process for cooling, indirect evaporative cooling of the air to less than half of the difference between the wet bulb temperature and then adiabatic cooling to the final temperature. With this process the temperature is reduced to less than the wet bulb temperature however the humidity is increased significantly. The high humidity limits their use to some residential and industrial applications where temperature not humidity is a concern.
There are two major problems with evaporative coolers that are to be used for supply of air directly for air conditioning: 1) associated high humidity, and 2) high cost of manufacturing.
There are several prior art indirect evaporative coolers that have a more thermodynamic efficient indirect evaporative cooling process than older versions. The prior art has not made it to manufacturing most likely due to their lack of understanding of materials needed to realize the efficiency of the process, the physical design of the apparatus that require at least two separate pieces of equipment and the resultant high cost to manufacture the designs.
This patent describes a method and apparatus for Dew Point Indirect Evaporative Cooling that utilizes a highly efficient thermodynamic process of heat and mass exchange between air and water, or other volatile fluid, is inexpensive to manufacture, providing temperatures that approach the dew point temperature of the entering fluid as opposed to the wet bulb temperature, and with little or no moisture added to the air.
Analogy: U.S. Pat. No. 4,002,040 (dated January 1977). Incorporates indirect evaporative cooling over a number of plates and several airflow designs that includes using some of the precooled dry air as evaporative air. This design requires a higher-pressure drop due to the need of the air to enter, pass through, exit an indirect evaporative cooler, then turn around 180 degrees and reenter the indirect evaporative cooler. The wetting system was proposed to be intermittent to help prevent over wetting, but this is difficult in practice as the drying is dependent on entering air humidity and flow rate. In addition the apparatus will not operate efficiently when either over wetted or partially dried out. The materials to build the apparatus were not discussed, but these affect not only the efficiency but also the manufacturability at a cost that could be desirable to consumers.
Russian Patent No. 2,037,104 (dated Jul. 7, 1991); U.S. Pat. No. 5,453,223 (filed Sep. 12, 1994), [though applicant does not admit the validity of the U.S. patent, as it was filed more than one year after the disclosure was public knowledge]. Disclosed using a square plate design with cross flow between the plate heat exchange surfaces. The plates were designed to have wet and dry zones with the opposing sides of the plate being dry and wet respectively. The wet and dry zones were created along the diagonal of the square plate where the air entered along a dry surface and proceeded to a wet surface where it departed. Because the plates had wet zones with opposing dry zones, the air passing in cross flow over them would be indirectly cooled. The design provided for pre-cooling of the air before entering an evaporative section and then the flow was split providing either waste heat exhaust air or cooling air to the user.
This prior art designs have the following disadvantages:
The existing method and design always increases the absolute humidity of air used.
In some applications higher humidity is not wanted.
The inefficiency of the working air""s lower exhaust temperatures indicating the loss of cooling capacity.
The unit""s inability to cool a product other than air.
The inability to separate the air streams entering the cooler for more efficient operation. Hot dry air coming out of a dehumidification process, separating from recirculating inside air.
The unit lacked flow direction in channels causing air mixing and therefore temperature mixing preventing the greatest possible temperature differences across a plate. This prevents the lowest possible outlet temperatures. The lack of flow direction also causes uneven or stagnated flow diminishes the effectiveness of the surface area of the plates.
Inefficient direct evaporative section of cooling air, caused by having an impervious surface on one side of the plate.
The concepts were never put into practical application most likely due to their complex design and expensive fabrication requirements.
The plates did not have a common wick material making it difficult to wet the plates.
The design did not allow for non horizontal wetting of panels or using natural capillary transporting of water to wet moist surface.
The design requires higher-pressure drops and thus impaired efficient surface area use.
The proposed method and apparatus for this invention eliminates these disadvantages.
The purpose of the method and apparatus for dew point indirect evaporative cooling is to provide the product fluid for example air, water, oil, etc., which is cooled by passing multiple product streams through the invention apparatus to a user. The apparatus uses multiple working air streams that are first precooled and then passed in cross flow, or counter flow, over an indirect evaporative cooling plate The working air streams, by evaporation, take heat from the heat exchange plate, which provides the interface between the working air and the product stream fluid, which in turn takes heat from the product fluid.
A further object is to obtain lower temperatures when air is used as the product fluid, by using an adiabatic evaporative section added after the indirect cooling dry section, creating an efficient direct evaporative process within the same apparatus.
Due to the thermodynamic cycle in the working air, a further refinement of the apparatus, air can be heated before entering the apparatus in some humid climate conditions to provide added cooling capacity by providing greater latent heat capacity. This may be done by direct heat input or by removing humidity from the entering air
Another object of this invention is to allow the humid working air exhaust to be used as the product and directed to the user for humidification of desired area, for example, in the winter, in residential areas.
The method and apparatus can be used in conjunction with existing desiccant dehumidification systems to cool building fresh makeup air taking advantage of relatively dry and cool building air making a very efficient process. It can also be used to cool dehumidified building recirculation air.
The plates in the apparatus are made of a layer of wick material with a thin waterproof or low permeability coating in dry zones. Channel guides or corrugated sheets can be used to hold the plates apart and give direction to the working air and product fluids.
The plate wick is wetted by wicking, or by natural capillary transportation of water out of a reservoir.